The biological function of DNA polymerase β in the whole organism was examined by using a transgenic Drosophila bearing chimeric gene consisting of a promoter region of Drosophila actin 5C gene and rat DNA polymerase β cDNA. Southern analysis of genomic DNA of transgenic flies and transmission pattern of pw+; polβ indicated that a single copy of pw+; polβ was integrated into each transgenic strain. To elucidate the role of DNA polymerase β in DNA repair, the sensitivity of transgenic strain to killing after treatment of MMC, MMS and UV were compared with that of host strain. The sensitivity of transgenic strain to mitotic recombination induced by MMC or MMS were also compared with host strain. The present results suggest that rat DNA polymerase β expressed in transgenic flies play an important role at least in the generation process of mitotic recombination.